Free radical polymerization of ethylene, with or without the presence of comonomers, produces low density polyethylene (LDPE). LDPE has excellent thermal processability but low mechanical strength due to its high concentration of long chain branches. Ziegler and single-site catalysts generally produce linear polyethylene which has little or no long chain branching. Linear polyethylene can be high density, medium density, or low density depending on the amount of longer chain α-olefins incorporated. Linear polyethylenes usually have superior strength at equivalent densities compared with LDPE. However, linear polyethylenes usually have unsatisfied thermal processability.
Methods to improve the processability of polyethylenes have been developed to allow for the easier processing of polyethylene products. One approach is to combine the polyethylene with a free radical initiator in a single or double-screw extruder. This process is generally performed at a temperature sufficient to melt the polyethylene and to allow the polyethylene to react with the peroxide, see, for example, U.S. Pat. Nos. 4,603,173 and 5,639,818. While thermal peroxidation in an extruder can result in long chain branching, which improves the melt strength, the process can also lead to unwanted crosslinking and gel formation.
It would be commercially useful to have a method of modifying polyethylenes that improves their processability without the undesirable side effects.